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BIG PHYSICS, BIG QUESTIONS –

GPS ‘thermometer’ could flag up climate change

By Kurt Kleiner

As GPS signals pass through the atmosphere, they are refracted slightly, with the angle of refraction depending on temperature and the amount of water vapour in the atmosphere. The signals are detected at satellites in low-Earth orbit (LEO)

(Image: GeoDAF)

The Global Positioning System could be used as a global thermometer and used to monitor climate change, say UK meteorologists.

The idea rests on a relatively new technique for taking atmospheric measurements, called GPS radio occultation.

This involves using a satellite in low-Earth orbit to receive signals from GPS satellites. As the signals pass through the atmosphere, they are refracted slightly, with the angle of refraction depending on temperature and the amount of water vapour in the atmosphere.

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Instruments on a number of research satellites measure GPS signals in this way, including the German CHAMP mission, and the joint US/Taiwan COSMIC mission.

These measurements are already used to help calculate the amount of water in the atmosphere, as well as temperature and density, which are useful in weather forecasting.

Man-made signal

Now two researchers at the Met Office Hadley Centre in Exeter and the European Centre for Medium-Range Weather Forecasts in Reading, both in the UK, suggest that the measurements of refraction might be used directly to confirm climate change.

In a paper to be published in Geophysical Research Letters, Mark Ringer and Sean Healy describe how they have used computer models to calculate the expected change in the refraction of the GPS signals as global warming continues.

Although natural atmospheric variations will also affect the measurements, they predict that, within 10 years, a strong signal of man-made climate change should be detectable. Their model indicates that radio waves going through the stratosphere will be bent through an angle 4% greater than today.

“I think it’s pretty significant,” says Robert Kursinski, an atmospheric scientist at the University of Arizona. “Here’s another way you can use this data in a way that will, in theory, point out a change in climate.”

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